Usually, ethylene terephthalate polyester fibers are obtained industrially by melt-spinning a polyethylene terephthalate having an intrinsic viscosity (hereinafter abbreviated as IV) of less than 1.2 at a temperature of not lower than its melting point, and then hot-stretching and heat-treating the resulting filaments.
Properties of ethylene terephthalate polyester fibers obtained by such a conventional process are such that the initial tensile modulus is about 160 g/d and the strength is about 9 g/d, even in the case of high tenacity fibers. IV of fibers is at highest about 1.0 (Seni Soran, Nippon Seni Kikai Gakkai ed., 1970).
On the other hand, for ethylene terephthalate polyester fibers to be used for industrial materials such as tire cords, ropes and the like, it is requested to impart improved properties such as a high modulus, a high tenacity, high fatigue resistance, high wear resistance and the like.
As one of studies for imparting such improved properties to ethylene terephthalate polyester fibers, Shimizu et al. disclose that fibers having a high melting point can be obtained by a melt spinning method wherein spinning is carried out at a high speed such as at the takeup speed of 6,000 to 7,000 m/min. to obtain fibers having the melting point of 268.4.degree. C. which is much higher than a melting point of conventional fibers [Seni Gakkaishi Vol. 33, No. 5, p 208 (1977): Vol. 34, NO. 2, p 43 (1978)]. However, the tensile modulus of the fibers is still low such as 80 g/d.
Then, as an important technique for imparting improved properties to polyester fibers, it is expected to develop a production technique using a high molecular weight ethylene terephthalate polyester wherein the polyester is highly stretched. Although it has been difficult to obtain a high molecular weight polyethylene terephthalate because a conventional polymerization of a polyethylene terephthalate is polycondensation and IV of a product is at highest about 1.8, it is possible to obtain an ultra-high-molecular-weight polyethylene terephthalate having IV of over 3.0 due to recent progress in a polymerization technique. Thus, there is a high possibility for imparting improved properties to polyethylene terephthalate polyester fibers.
However, when a melt spinning method is employed to impart improved properties to an ultra-high-molecular-weight polyethylene terephthalate, it is very difficult to spin the polymer by a conventional melt spinning technique because a melt viscosity is very high due to the ultra-high-molecular-weight of the polymer and fluidity of a melt thereof is very low. For that reason, a spinning device which can withstand high pressure has been proposed and spinning under high pressure and high shear conditions has been studied (Japanese Patent Kokoku No. 48-19887, Japanese Patent Kokoku No. 47-33727 and U.S. Pat. No. 3,846,377). However, ethylene terephthalate polyester fibers having desired improved properties have not yet been obtained.
On the other hand, Seizo et al. disclose a study for possibility of improvement of properties by increasing a molecular weight of stretched fibers obtained from a common polyethylene terephthalate having IV of less than 1.2 by solid phase polymerization [Seni Gakkaishi, Vol. 35, No. 8, p 328 (1979)]. As the result, stretched polyethylene terephthalate fibers having a high melting point such as 276.degree. C. is obtained. However, the initial tensile modulus of the fibers is remarkably lowered from 50 g/d to 20 g/d. Therefore, although a melting point of the fibers becomes higher according to this method, the fibers having a high modulus in addition to the high melting point is not yet obtained.
In general, the properties required for fibers for industrial materials such as tire cords which reinforce rubber have desirably a high tenacity and a high modulus. However, polyethylene terephthalate fibers which are presently used for tire cords have the tensile strength of 9 g/d and the tensile modulus of 130 to 150 g/d. In general, polyethylene terephthalate fibers having a tensile modulus of less than 130 g/d are not used because they have less reinforcing effect on rubber.
Since the tensile strength of a polyethylene terephthalate fibers for tire cords is about 1 g/d lower than that of nylon 6 or nylon 66 fibers for tire cords and it has been requested to improve the strength thereof.